Gas chromatographic (GC) detectors are based on a variety of physical and chemical processes. Some of the most widely used GC detectors function by converting chemical species in the GC effluent to gas-phase ions, which can then be collected and directly transduced to an electrical signal. For example, the flame ionization detector (FID) is the most universally applied GC detector, because of its high sensitivity, wide dynamic range, and ability to detect a variety of organic compounds. However, the FID is a nonspecific detector that responds, in general, to all chemical compounds that contain carbon.
Particularly when the compound of interest cannot be easily resolved above background, it may be advantageous to employ a selective detector that responds only to a few limited compounds with little or no response to interfering compounds. A thermionic detector (TID), as known as a nitrogen-phosphorous detector (NPD), is such an element-specific ionization detector. The TID relies on the specific ionization of the sample compound near a hot thermionic surface, resulting from the high selectivity of surface ionization with respect to the ionization potential of the sample compound. Therefore, the TID can provide very specific ionization of sample compounds containing electronegative functional groups, such as nitrogen, phosphorous, halogen atoms, and sulfur. A conventional TID typically has a ratio of 104:1 selectivity over carbon. Because of its sensitivity and selectivity for nitrogen and phosphorus, the TID is especially useful for the analysis of pesticides, chemical warfare agents, explosives, pharmaceuticals, and other organic compounds that contain nitrogen or phosphorus.
In FIG. 1 is shown a conventional TID 10 of the type used with a laboratory-based analytical system. The TID is similar to a FID, except that a rubidium- or cesium-containing bead 11 (e.g., a rubidium silicate bead) is formed on a heater coil 12 (e.g., a platinum wire). The bead 11 is electrically heated by running a current through the coil 12 to provide a thermionic source. The temperature of the thermionic source can affect the sensitivity of the detector, since the source must remain hot enough to produce a reactive chemical environment. Typically, the source is heated to between 600 and 800° C. The bead 11 is situated above a jet 13, through which passes a sample gas (e.g., containing a nitrogen or phosphorous compound) 15 that can be mixed with hydrogen 16. Make-up gas (e.g., air or oxygen) 17 can also be introduced to dilute the hydrogen mixture. Typical flow rates for a conventional TID are 2 to 6 mL/min of H2 and 60 to 200 mL/min of air, dependent on the interior volume of the detector. The negative ions produced by thermochemical reactions that occur when the gases impinge on the low work function surface of the hot bead 11 are collected by a positively biased collector electrode 18 and counted with a sensitive electrometer circuit. A typical circuit can detect 0.4 pg of nitrogen and 0.1 pg for phosphorus. See R. P. W. Scott, Chromatographic Detectors, Marcel Dekker (1996).
Laboratory-based analytical systems can provide precise and accurate results, however, the time between sample collection in the field and the availability of results from the laboratory can often be weeks. Portable, handheld microanalytical systems, which have been termed “chemical laboratories on a chip,” are being developed to enable the rapid and sensitive on-site detection of particular chemicals, including pollutants, high explosives, and chemical warfare agents. Preferably, these microanalytical systems should provide a high chemical selectivity to discriminate against potential background interferents and the ability to perform the chemical analysis on a short time scale. In addition, low electrical power consumption and reagent usage is needed for prolonged field use. See, e.g., Frye-Mason et al., “Hand-Held Miniature Chemical Analysis System (μChemLab) for Detection of Trace Concentrations of Gas Phase Analytes,” Micro Total Analysis Systems 2000, 229 (2000).
Although conventional TIDs are now widely used in laboratory-based analytical systems, there remains a need for a microfabricated TID that can be used with microanalytical systems. Typically, the microfabricated TID can be combined with a microfabricated separation column and used in gas chromatography analysis to detect the nitrogen and/or phosphorous content in analytes eluted from the column.